Method for glueing together disc elements

ABSTRACT

A method and a device for glueing together two disc elements. The two disc elements are brought together coaxially against each other to form a gap between the disc elements. In an inner area of the gap a liquid adhesive is applied so that it comes essentially simultaneously into contact with facing sides of the two disc elements. The two disc elements are then brought towards each other to achieve a uniform adhesive layer between the disc elements. The disc comprises lower and upper holders for respective disc elements and a unit for applying liquid adhesive in an inner area of the gap.

[0001] The present invention relates to a method of glueing together twodisc elements, especially substrates for digital audio, video orcomputer discs, such as digital video discs (DVDs).

[0002] When manufacturing certain types of data storage discs, such asso-called DVDs, with one or more information layers, two substrate discsof polycarbonate of a thickness of 0.6 mm are normally glued together,with the information layer(s) adjacent to the adhesive layer joining thediscs.

[0003] In one proposed method of glueing the adhesive is applied to oneof the discs by means of a printing process, for example screenprinting, whereafter the discs are joined together. This method does notwaste adhesive and the application can be done very rapidly. However,there is some risk of air enclosures when the discs are put together andsome risk that the printing system will break down should the screenprinting blanket rupture.

[0004] A method using double adhesive tape for joining the two discelements has also been suggested, but this method causes air enclosuresand provides a joint of unreliable strength and thermal stability. Norcan this method be used for DVDs of the type “SD9”, i.e. where both ofthe discs elements have information layers and where these layers are tobe read by laser light which must pass, with the correct defraction,through the joint layer between the disc elements.

[0005] One purpose of the present invention is to suggest a glueingmethod which prevents the occurrence of air enclosures in the adhesivelayer between the discs.

[0006] For this purpose, the method according to the invention ischaracterized by the steps of:

[0007] a) bringing the two disc elements together coaxially with eachother in such a manner that a gap is left between the disc elements:

[0008] b) applying, in an inner area of the gap, a liquid adhesive insuch manner that the adhesive will essentially come into contactsimultaneously with facing sides of the two disc elements, and

[0009] c) bringing the disc elements towards each other to achieve awell defined uniform filling out of adhesive between the disc elements.

[0010] It has been found that a simultaneous or essentially simultaneouswetting of the opposing gap surfaces with an easily flowing adhesivewith good capillary filling properties is an essential precondition inorder for the applied adhesive to flow out and be evenly distributedover the entire surface between the outer and inner peripheral limitingedges of the disc elements without forming air enclosures, since thedisc elements are brought towards each other with only the aid ofgravity and “are sucked together” by the capillary forces of theadhesive. In this way a very uniform, homogeneous adhesive layer withoutair enclosures can be obtained without forceably pressing the discstogether.

[0011] The disc elements can be kept plane-parallel to each other duringthe application of adhesive, or they can be held relative to each otherin such a way during the application of adhesive that a radiallyoutwardly diverging gap is formed between at least one radially outersection of the disc elements.

[0012] The diverging gap can be obtained by curving one or both of thedisc elements convexly from each other, preferably by creating asubatmospheric pressure on the side of the disc element facing away fromthe gap.

[0013] The adhesive is applied preferably by injection via a nozzleinsertable into the gap during simultaneous rotation of the discelements, whereupon a coherent annular adhesive string can be laid outin the gap between the disc elements.

[0014] After application of the adhesive string, the nozzle is retractedand the two disc elements are rotated individually in oppositedirections, each at least half a rotation, preferably about one half toone rotation, so that any uneveness in the circumferential distributionof the adhesive string can be evened out by the disc elements producinga shearing effect in the adhesive. The result will be a homogeneous anduniform ring of adhesive, which is an essential precondition for evendistribution over the entire disc surface of the adhesive in asubsequent step.

[0015] The present invention also relates to a device for glueingtogether disc elements of the above mentioned type, said devicecomprising a lower holder for carrying one disc element, an upper holderfor carrying the other disc element in a coaxial relation to the lowerdisc element to form a gap between the disc elements, and a means forapplying a liquid adhesive in an inner area of the gap.

[0016] In order to create a diverging gap to facilitate insertion of theadhesive application means between the disc elements, there is oneholder that can have an essentially convex surface facing the discelement. This convex surface is intended to define, together with thedisc element and sealing elements a space which can be subjected tovacuum. Alternatively, the holders can be disposed to hold the discelements plane-parallel.

[0017] The means for applying adhesive can comprise a nozzle which isdisposed to direct, either radially from outside or inside, a stream ofadhesive into the gap in such a way that the stream of adhesive strikesfacing surfaces of the disc elements essentially simultaneously. Thenozzle can be insertable into and extractable from the gap so that itsopening comes close to the facing cap surfaces in an inner portion ofthe gap.

[0018] In order to provide a continuous annular string of adhesive inthe gap, the holders are rotatable.

[0019] For the case when only one of the holders is arranged to curve anassociated disc element to create the adhesive introduction gap, it issuitable to make the lower holder with a planar disc-carrying surface.

[0020] The invention will be described in more detail below withreference to the accompanying drawings, in which:

[0021]FIG. 1 shows a schematic cross-section of an enlarged section of aso-called DVD of “SD9” type to illustrate its construction;

[0022]FIG. 2 shows a schematic cross-sectional view of a deviceaccording to the invention for glueing together DVDs substrates, whereboth disc substrates are curved to form an adhesive introduction gap;

[0023]FIG. 3 shows the device in FIG. 2 in a step after injection ofadhesive, when the disc elements are flattened out and the homogeneouslayer of adhesive has been formed therebetween;

[0024]FIG. 4 shows an alternative embodiment of the device according tothe invention, where only one holder curves its associated disc elementto form an adhesive introduction gap;

[0025]FIG. 5 shows the device in FIG. 4 when the disc elements have beenjoined together by gravity and the capillary forces of the adhesive;

[0026]FIG. 6 shows an additional embodiment of the device according tothe invention, where the disc elements are kept in separatedplane-parallel relation during adhesive application, whereby theadhesive is introduced by means of an adhesive introduction nozzleinsertable radially from within into the cap between the disc elements;and

[0027]FIG. 7 shows the device in FIG. 6 in a step after the introductionof adhesive, when the upper disc element has been released from itsholder after having been joined to the lower disc element to form ahomogeneous adhesive layer between the disc elements.

[0028]FIG. 1 shows schematically a digital data storing disc of CDformat, namely a so-called digital video disc 10 of the type called“SD9”. Such a disc 10 comprises two disc substrates 12, 14, each havinga thickness of 0.6 mm and a diameter of ca 120 mm and made of atransparent plastic material such as polycarbonate. Each disc substrate12, 14 respectively has an information layer 16, 18 respectively onfacing sides containing short 20 and long 22 depressions with a depth ofabout 0.1 μm. The information layer 16 on the lower disc substrate 12 iscoated with a layer of for example Al, Au or SiN which issemi-reflecting for laser light, while the information layer 18 on theupper disc substrate 14 is made completely reflecting. The two discsubstrates 12, 14 are joined together by means of a thin adhesive layer24 (cirka 14-70 μm), the index of refraction of which should becomplementary to the index of reflection of the substrates(polycarbonate) in order to avoid any refractive error when the laserlight is to read the upper information layer 18. In FIG. 1, a light beam26 is shown which is focused on the lower, semi-reflecting layer 16.

[0029] For DVDs 10 with double information layers, as shown in FIG. 1,it is of the greatest importance that the adhesive layer 24 behomogeneous and completely free of air enclosures in order to provideundisturbed focusing of the laser light on the information layer 18.This problem is, however, not critical in a single layer DVD (“SD5”),where the upper disc substrate can be a blank information free disc,i.e. where only a single lower completely reflecting layer is read.

[0030]FIG. 2 shows schematically a device 28 according to the inventionfor glueing together two disc substrates 12, 14. The device 28 comprisesa lower substrate holder 30 and an upper substrate holder 32, which arerotatable about a central shaft 34. The holders 30, 32, which can beessentially identical, each have an outer and inner sealing ring 36 and38 respectively and are formed with a thicker hub portion 40 and athinner peripheral portion 42 in order to create between the holders 30,32 a radially outwardly diverging space 44 (FIG. 3) in order to make itpossible to hold the disc substrates 12, 14 in a convexly curved statefor the application of adhesive, as shown in FIG. 2.

[0031] For the purpose of holding securely the respective disc elements12, 14 in a convexly curved state to form an intermediate radiallyoutwardly diverging gap 46 for the introduction of adhesive, each of theholders 30, 32 have a surface section between its outer and innersealing rings 36, 38, forming therebetween and with a disc element aspace 48 which is in communication with a vacuum source (not shown).After placing a disc substrate 12, 14 on each holder 30, 32 andconnecting a vacuum source (not shown), the substrates can be made tocurve to the shape shown in FIG. 2 or similar thereto, thus making itpossible to insert an adhesive dispensing nozzle 50 into the outerbroadest portion of the gap 46 in order to dispense a carefully measuredamount of a flowable adhesive towards a narrow portion of the gap sothat the adhesive essentially comes into contact simultaneously with thefacing surfaces of the substrates. By rotating the two holders 30, 32one rotation at the same time as adhesive is injected, an annular stringof adhesive 52 is created around the substrate gap. After application ofthe adhesive string 52, the nozzle 50 is retracted from the gap 46. Inorder to eliminate any uneveness in the distribution of adhesive in theperipheral adhesive string, the two holders are also rotatable relativeto each other, so that the adhesive string can be subjected to ashearing effect and thus be made more even before the disc substratesare brought together. The relative rotation is performed suitably bycausing the two holders to rotate approximately one half to one completerotation in opposite directions. The vacuum in the space 48 is thenequalized so that the disc substrates 12, 14 are released from theirrespective holders and spring back to a normal flat state, as shown inFIG. 3, whereupon the adhesive in the string 52, due to the bringingtogether of the disc substrates and the capillary forces of theadhesive, will be distributed uniformly in the space between the discsubstrates 12, 14. Any air present between the disc substrates will thusbe expelled from the intermediate space, both radially inwards andradially outwards so that a thin even adhesive layer is formed which isfree from air enclosures.

[0032]FIG. 4 shows an alternative simplified embodiment of a discglueing device according to the present invention, which differs fromthe device described above only in that the lower holder 30′ is madeflat and lacks means for bending the associated disc substrate. Instead,only the upper holder 32′ is arranged to achieve an adhesiveintroduction gap 46 and for this purpose it must bend its associateddisc substrate 14 somewhat more than in the embodiment according toFIGS. 2 and 3 to permit insertion of the adhesive application nozzle 50.

[0033]FIG. 5 shows the disc elements 12, 14 in their flat state afteradhesive application in the embodiment shown in FIG. 4, after theadhesive has spread out and filled the gap between the disc elements 12,14 to the required extent.

[0034] An additional conceivable embodiment of the device according tothe invention is shown in FIG. 6, where the disc elements 12, 14 at glueapplication are kept essentially in a plane-parallel spaced state by aplanar lower holder 30″, and a planar upper holder 32″. The adhesive isinserted into the gap 46 between the disc elements of means of anadhesive injection nozzle 50′, which can be inserted into the gapradially from within. After application of a circular string 52 ofadhesive by rotation of the holders 30″, 32″ one rotation duringadhesive dispensing, the nozzle 50′ is then retracted, and after arelative rotation of the holders for even spreading of the adhesivestring the upper disc element 14 is released from its holder 32″, asshown in FIG. 7, and the disc elements 12, 14 are brought together bythe effect of gravity on the upper disc element and the effect ofcapillary forces in the adhesive, striving to pull the two disc elements12, 14 towards each other as the adhesive spreads out and fills thespace between the disc elements forming a uniform adhesive layer freefrom air enclosures.

[0035] Within the scope of the invention it is also conceivable to applythe adhesive by injecting it into the gap via one or more fixed nozzles,which do not need to be inserted into the gap. It would thus besufficient to have such a fixed nozzle which ejects a well-definedstream of adhesive towards a preferably tapered portion of the gap 46,making contact at the same time with the two facing surfaces of thesubstrates.

[0036] It is also conceivable to make the device in such a manner thatthe disc elements form a radially inwardly diverging gap when anadhesive introduction nozzle of the type shown in FIGS. 6 and 7 is used.

[0037] It is also conceivable within the scope of the invention to makethe substrate holders in such a manner that, after adhesive application,they also exert a pressure on the disc substrates 12, 14 to acceleratethe filling out of the gap with adhesive.

What is claimed is:
 1. Method of glueing together two disc elements, fordigital audio, video or computer discs, which comprises the steps of: a)bringing the two disc elements together coaxially with each other insuch a manner that a gap is left between the disc elements; b) applyingin an inner area of the gap, while rotating the disc elements, a liquidadhesive in the shape of a ring in such manner that the adhesive willessentially come into contact simultaneously with facing sides of thetwo disc elements; c) rotating the disc elements relative to each otherto even out the adhesive ring by shearing action; and d) bringing thedisc elements towards each other to achieve a well-defined, uniformfilling out of adhesive between the disc elements.
 2. Method accordingto claim 1, wherein the disc elements are each rotated at least one halfrotation in opposite directions.
 3. Method according to claim 1, whereinthe disc elements are each rotated between one half to one completerotation in opposite directions.
 4. Method according to claim 1, whereinat least one adhesive injection nozzle is inserted into the gap prior tothe application of adhesive.
 5. Method according to claim 4, wherein thenozzle is inserted into the gap radially from the outside.
 6. Methodaccording to claim 1, wherein the disc elements are kept in parallelrelationship to each other during the application of adhesive.
 7. Methodof glueing together two disc elements, for digital audio, video orcomputer discs, which comprises the steps of: a) bringing the two discelements together coaxially with each other; b) holding at least one ofthe disc elements in a convexly curved shape relative to the other discelement to form a radially outwardly diverging gap between at least aradially outer section of the disc elements; c) applying, in an innerarea of the diverging gap, while rotating the disc elements, a liquidadhesive in the shape of a ring in such manner that the adhesive willessentially come into contact simultaneously with facing sides of thetwo disc elements; and d) bringing the disc elements towards each otherto achieve a well-defined, uniform filling out of adhesive between thedisc elements.
 8. Method according to claim 7, wherein at least oneinjection nozzle is inserted into the diverging gap prior to adhesiveapplication.
 9. Method according to claim 7, wherein the injectionnozzle is inserted into the diverging gap radially from outside theouter section of the disc elements.
 10. Method according to claim 7,wherein at least one of the disc elements is curved against a respectiveprofiled holder by creation of a subatmospheric pressure on the side ofthe disc element facing the holder.
 11. Method according to claim 7,wherein the disc elements, after the application of adhesive, arerotated relative to each other to even out the adhesive ring by shearingaction.
 12. Method according to claim 7, wherein the disc elements areeach rotated at least one half rotation in opposite directions. 13.Method according to claim 7, wherein the disc elements are each rotatedbetween one half to one complete rotation in opposite directions.